Abstract: Radiographic evidence of OA is present in more than 50% of patients 10 years after ACL reconstruction. However, the etiology of early onset OA after ACL reconstruction is not well understood. While previous work has demonstrated changes in joint motion following reconstruction, it is unclear how these altered motions relate to changes in the local mechanical response of cartilage to in vivo loading. This information may be critical to understanding the onset of OA, as the mechanical response of cartilage affects cartilage homeostasis. In line with this mechanism, our pilot data indicates that regions of high cartilage strain measured in patients with ACL injury and reconstruction are susceptible to decreased cartilage thickness, a characteristic feature of OA. Importantly, these degenerative changes are observed as early as 18 months post-reconstruction. Therefore, in this proposal our overall hypothesis is that site-specific changes in the mechanical response of cartilage to loading following ACL injury and reconstruction are predictive of long-term cartilage degeneration. Specifically, we hypothesize that in regions of cartilage experiencing elevated strain during loading, early degeneration will be reflected by altered composition and decreased cartilage thickness. Thus, we will measure localized cartilage strain, composition, and thickness at four time-points: after ACL injury but prior to reconstruction, as well as at 3 months, 1 year, and 2 years after surgery. At each time point, a combination of high-speed biplanar radiography and MR imaging will be used to measure local cartilage strains during in vivo loading. To assess cartilage degeneration, we will use MRI to measure site-specific changes in composition (using T1rho and T2 relaxation times) and cartilage thickness. Additionally, our analyses will account for relevant biological variables such as age, sex, and BMI, and clinical factors such as graft placement characteristics and meniscus injuries. In addition to mechanical factors, biological factors such as joint inflammation and lubrication may also play a role in cartilage degeneration after ACL reconstruction. Therefore, synovial fluid and serum will be collected to measure inflammatory mediators and metabolic biomarkers. Using this data, we will develop a predictive model of cartilage degeneration after ACL reconstruction that utilizes both mechanical and biological factors, as well as other demographic and clinical characteristics, to predict declines in cartilage health. Furthermore, this comprehensive dataset will be used to develop clinical phenotypes to identify those at high risk for cartilage degradation after ACL reconstruction. Importantly, the development of these phenotypes will enable targeted treatment approaches focusing on surgical procedures, pharmaceutical targets, and non- pharmacological interventions such as physical therapy or weight loss in preventing cartilage degeneration. Therefore, our findings will both elucidate the role of alterations in the local mechanical response of cartilage on degeneration after ACL reconstruction and improve the identification and treatment of patients at high risk for cartilage degeneration.